Turbochargers may supply one or more cylinders of an internal combustion engine with combustion air which is compressed. A turbocharger comprises a turbine and a compressor which can have a similar construction and are mounted on a joint shaft. A mass flow of an exhaust gas may rotate a turbine wheel in the turbine in order to turn the compressor. Torque is transmitted via the joint shaft to the compressor wheel in the intake tract, as a result of which the compressor compresses combustion air.
As long as sufficient exhaust gas flows into the turbine, the rotational speed may be sufficient to turn the compressor to a desired compressor speed. However, providing sufficient exhaust gas to the turbine at lower loads presents certain challenges. The turbo may react in a delayed manner (e.g., turbo lag) in response to a sudden acceleration at low engine loads.
Two turbochargers can be used in the upper rotational speed range, while in the lower rotational speed ranges in the case of only one used turbine, said turbine can build up a charging pressure more rapidly.
Problems in using sequential twin turbochargers may include control of torque fluctuations during activation or deactivation of the second turbocharger and corresponding power fluctuations, as well as a costly exhaust gas flap valve control mechanism in order to activate or deactivate the second turbocharger. Moreover, even when using sequential twin turbochargers, a part of the exhaust gas energy is still unused from the continuously driven turbocharger, and control of a low-pressure and/or high-pressure exhaust gas return is difficult. The inventors herein have recognized that a sequential twin turbocharger which enables effective turbocharging of an internal combustion engine while avoiding the cited disadvantages is possible. To resolve the above described problem at low loads, two turbochargers may be used in parallel. More specifically, the two turbochargers may be sequential, wherein a first turbocharger is continuously driven during engine operation and a second turbocharger is only driven during engine conditions leading to insufficient exhaust flow to the turbine of the first turbocharger.
In one example, the issues described above may be addressed by a system for a turbocharged engine with a first turbocharger comprising at least a first turbine and a first compressor, and a second turbocharger comprising at least a second turbine and a second compressor, where the first and second turbochargers are arranged in parallel, an exhaust line comprising a valve device located between the engine and the second turbine, and a first electrical energy converter located on a second shaft between the second turbine and the second compressor. In this way, the second compressor may be able to assist the first compressor during periods of insufficient exhaust flow.
As one example, the first turbine may further include a wastegate with a bypass passage leading to the second turbine and as a result, normally unused exhaust gas may be used to drive the second turbine. Additionally, the first turbocharger may include a second electrical energy converter located on the first shaft between the first turbine and the first compressor. The first and second electrical energy converters may be operated based on one or more of an exhaust flow and a battery state of charge.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.